Methods for producing gasoline boiling range aromatic hydrocarbons from nonaromatic hydrocarbon feeds by employing a medium pore size zeolite type catalyst are generally known as exemplified in. e.g., U.S. Pat. Nos. 3,760,024, 3,843,741 and 4,350,835. In such processes, the desired end product comprises primarily gasoline boiling range materials. Gasoline, as such term is used herein, and as such term is commonly used in the petroleum industry, is useful as a motor fuel for internal combustion engines. More specifically, gasoline is hydrocarbon in nature, and contains various aliphatic and aromatic hydrocarbons having a full boiling range of about 280.degree. to 430.degree. F., depending on the exact blend used and the time of year. Although gasoline is predominantly hydrocarbon in nature, various additives which are not necessarily exclusively hydrocarbon are often included. Additives of this type are usually present in very small proportions. e.g., less than 1% by volume of the total gasoline. Further, it is also not uncommon for various gasolines to be formulated with non-hydrocarbon components, particularly alcohols and/or ethers as significant, although not major, constituents thereof. Such alcohols, ethers and the like have burning qualities in internal combustion engines which are similar to those of hydrocarbons in the gasoline boiling range. For purposes of this specification and the present invention however, the term "gasoline" denotes a mixture of hydrocarbons boiling in the aforementioned gasoline boiling range and is not intended to include the above-referred to additives and/or non-hydrocarbon constituents.
High octane gasoline is desirable for use with internal combustion engines from a standpoint of fuel efficiency, and thus also is attractive from an economic perspective. Further, the gradual phasing out of lead in gasoline has created a demand for new methods for obtaining high octane gasoline. It is known that aromatic gasoline boiling range hydrocarbons have high octane (R+O). (M+O) and/or (R+M)/2 values. It is known that gasoline octane is related to the aromatic selectivity of the catalyst. An increase in aromatic selectivity will result in increased gasoline octane values. Aromatic selectivity, as used throughout the specification, is defined as (wt % aromatics produced/C.sub.5 +).times.100. Hence methods which are capable of increasing the aromatic selectivity of the catalyst are very desirable.
Hydrogenolysis is an unwanted side reaction which occurs during the production of gasoline and which reduces the aromatic selectivity of the catalyst. A survey of the literature shows that noble metal/SiO.sub.2 or Al.sub.2 O.sub.3 catalysts, when modified with sulfur, silver, tin, and copper are known to have different hydrogenolysis activity than unmodified noble metal/SiO.sub.2 or Al.sub.2 O.sub.3 catalysts. See, e.g., P. G. Menon et al., "Effect of Sulfur Poisoning on the Hydrogenolysis Activity of Pt in Pt/Al.sub.2 O.sub.3 Catalysts". Ind. Eng. Chem. Prod. Res. Dev., 21, 52 (1982) C. R. Apesteguia et al. "The Role of Catalyst Presulfurization in Some Reactions of Catalytic Reforming and Hydrogenolysis", J. of Catalysis, 78 352 (1982): P. Biloen et al. "The Role of Rhenium and Sulfur in Platinum-Based Hydrocarbon-Conversion Catalysts", J. of Catalysis, 63, 112 (1980): J. R. H. van Schaik et al, "Reactions of Alkanes on Supported Pt-Au Alloys", J. Catalysis, 38, 273-282 (1975); V. Ponec. "Selectivity in Catalysis by Alloys", Cat. Rev. Sci. Eng., 11 41 (1975): and F. M. Dautzenberg et al, "Conversion of n-Hexane over Monofunctional Supported and Unsupported PtSn Catalysts", J. of Catalysis, 63. 119 (1980).
Although it is well known that sulfur compounds are capable of reducing the hydrogenolysis activity of noble metals supported on amorphous supports (see e.g., Menon et al supra), the response of zeolite supported noble metal catalysts to sulfur poisons is not at all predictable. For example, with respect to noble metal supported on amorphous (non-zeolite) supports, both geometric and chemical modifications of platinum have been proposed to explain the resulting change in reactivity upon contact with H.sub.2 S, SO.sub.2 or organic sulfur compounds. However it is difficult to distinguish the two mechanisms experimentally. The geometric argument proposes that hydrogenolysis requires large ensembles of platinum atoms. Hence, if platinum is diluted with sulfur atoms the platinum particle size decreases and distance between platinum atoms increases, resulting in decreased hydrogenolysis activity, on the other hand, the chemical argument suggests that Pt--S has different (i.e. higher in some instances, lower in others) reactivity for hydrogenolysis compared to Pt alone because of its different structure.
Rabo et al, in "Sulfur-Resistant Isomerization Catalyst: Study of Atomic Platinum Dispersions On A Zeolite Support", Third International Congress Catalysis, North Holland, Amsterdam, 1965, Vol. 2. 1329, disclose that highly dispersed platinum within zeolite Y, which is a large pore size zeolite, demonstrated high resistance to sulfur poisons, whereas when zeolite Y was impregnated with platinum on the outside surface in anionic form, the catalyst composition rapidly lost activity in the presence of thiophene. Hence, Rabo et al clearly teach the unpredictable responses of noble-metal/zeolite catalysts to sulfur.
It has also been theorized that the platinum particle size and the presence of a metal modifier play a role in the sulfur sensitivity of noble metal/zeolite catalysts. See. e.g., P. Gallezot et al "Unusual Catalytic Behavior of Very Small Platinum Particles Engaged In Y Zeolites" Proceedings of the Sixth International Congress On Catalysis, Chemical Society, London. 2, 696 (1977): T. M. Tri et al "Sulfur Resistance of Modified Platinum Y Zeolite". Studies In Surface Science and Catalysis. 5, 279 (1980). However, both of these studies also point out the undesirability of organic sulfur compounds in the feed when using zeolite Y type catalysts. The prior art as a whole would suggest, if anything, that the sulfur sensitivity of zeolite catalysts is unpredictable, and in many instances that the presence of sulfur in the feed is not desirable.
Further, it is also known that other large pore size zeolite catalysts, such as zeolite L type catalysts e.g., Pt/Ba/zeolite L or Pt/K/zeolite L are very sensitive to sulfur and the feed must contain less than 0.05 ppm weight of H.sub.2 S. On the other hand. U.S. Pat. No. 4,579,831 teaches that binding Pt/Ba/L or Pt/K/L (e.g. employing a matrix type binder which forms a catalyst composite with the zeolite) will improve the sulfur resistance. This also demonstrates the unpredictable nature of zeolite catalysts to sulfur poisons.